This invention relates generally to processes for aerating suspensions. More particularly, the present invention relates to processes for flotating suspensions, for example in deinking of pulp suspensions, with one or more stages and/or cells, where each stage/cell has its own separate liquid loop. In addition, the invention relates to a device for implementing the process.
Re-use of printed papers containing a high concentration of minerals (pigments, such as calcium carbonate and kaolin) requires process adjustments in the recycling process and in repulping during the deinking process.
For the recycling process, the input of non-fibrous substances (printing ink, minerals, etc.) involved is problematical. Operators of waste-paper treatment plants are forced to target removal of excess printing ink and mineral substances. Deinking flotation is a process for removing contaminants and printing ink particles from fibre pulp suspensions, particularly those produced in waste-paper treatment. Since it uses air bubbles to remove the ink particles, this process has only limited suitability for targeted removal of dirt specks at the same time, together with efficient ash removal and/or reduction of fines at low fibre loss.
Hydrophobic solids, or solids that have been rendered hydrophobic by means of surface-active substances, such as printing ink particles, etc., are carried to the surface of the liquid by the air bubbles adhering to them and can be removed there as foam. Depending on process management, dirt specks and fillers are removed more or less non-specifically along with the hydrophobic solids.
Examples of conventional processes are provided in EP 1 124 001 A2 or EP 1 262 593 A1, where the individual flotation stages can also be designed according to U.S. Pat. No. 4,255,262, EP 0 243 690, DE 31 20 202, or U.S. Pat. No. 6,208,549, for example. Processes of this type are used to remove contaminants and printing ink particles from pulp suspensions, particularly those produced in waste paper treatment. By applying this type of process, non-specific removal of mineral components (e.g. fillers, etc.) is achieved because their chemical surface properties (particularly loading) are such that they react differently to the ink particles in the pulp suspension, which are relatively easy to render hydrophobic. In addition, the specific weight of the mineral components is approximately two to three times higher than that of the ink particles. In the processes known, however, some disadvantages occur in selective flotation. If, for example, we try to target the removal of fillers by making certain changes to the process, this always results in a drop in the efficiency of printing ink and dirt speck removal, thus there is a need for additional changes in gas loading, adjustments to the raw material fluctuations, and changes in throughput. Furthermore, if the overflow quantity is increased in order to raise the removal rate of dirt specks and mineral substances, the fibre loss rises in equal proportion to this, which leads to greater expense for disposal and raw materials required. Dirt specks are very difficult to remove from the flotation process because of their particle size and the flow resistances prevailing in the tank.